(1) Field of the Invention
The invention relates to a method of reducing surface reflectivity and eliminating directional reflection lights from substrates during photolithography processes in the manufacture of integrated circuits.
(2) Description of the Prior Art
Referring to FIG. 1, there is shown a portion of a partially completed integrated circuit in which there is a silicon substrate 10. Field oxide regions 12 are formed in and on the silicon substrate resulting in a uneven topography of the top surface of the substrate. A layer 14 of polysilicon is deposited over the surface of the substrate. A layer of photoresist 16 coats the surface of the polysilicon layer 14 and planarizes the substrate. When the photoresist is exposed to actinic light, light waves 18 will reflect off the polysilicon surface onto the unexposed portion of the photoresist 20. After the photoresist is developed, as shown in FIG. 2, notches 22 will be found in the resulting photomask. These notches 22 are caused by directional reflection lights.
Anti-reflection coatings (ARC) have been used in semiconductor manufacturing to suppress reflection lights during the masking process. The use of ARC""s allows for the definition of sharp images and the avoidance of resist notching. However, these additional ARC layers are expensive, require additional process steps, and difficult to control in manufacturing.
U.S. Pat. No. 5,324,689 to Yoo teaches a different method of avoiding resist notching. Yoo uses a spin-on-glass layer underlying the photoresist layer. This allows the photoresist layer to have a uniform thickness, thus avoiding the reflection problem caused by the differing photoresist depths.
Accordingly, it is a principal object of the present invention to provide an effective and very manufacturable method of avoiding resist notching during photolithography across an uneven topography.
Another object of the present invention is to reduce surface reflectivity of surfaces having an uneven topography.
A further object of the present invention is to eliminate directional reflection lights from substrates.
Yet another object is to create micro light traps which reduce surface reflectivity and eliminate directional reflection lights from substrates.
In accordance with the objects of this invention a new method of avoiding resist notching in the formation of a polysilicon gate electrode in the fabrication of an integrated circuit device is achieved. Bare active areas are provided surrounded by field oxide isolation on a semiconductor substrate wherein the surface of the substrate has an uneven topography due to the uneven interface between the active areas and the isolation. A polysilicon layer is deposited over the active areas and the field oxide isolation of the substrate. The surface of the polysilicon layer is roughened using a plasma etching process wherein pits are formed on the surface which act as light traps. The roughened polysilicon layer is covered with a layer of photoresist. Portions of the photoresist layer are exposed to actinic light wherein reflection lights from the actinic light are trapped in the pits. The reflection lights do not reflect onto the unexposed portion of the photoresist layer. The photoresist layer is developed and patterned to form the desired photoresist mask for the polysilicon layer wherein the absence of reflection lights reflecting onto the unexposed portion of the photoresist results in the notch-free photoresist mask in the formation of a polysilicon gate electrode in the fabrication of an integrated circuit device.